1. Field of the Invention
The present invention relates generally to a CDMA mobile communication system, a searcher circuit and a communication method. More particularly, the invention relates to a CDMA mobile communication system, a searcher circuit and a communication method realizing a Code Division Multiple Access (CDMA) communication by combining outputs of a finger circuit performing dispreading for a reception signal.
2. Description of the Related Art
In CDMA mobile communication which has been expected as a mobile communication in the next generation, a radio wave reaching to a mobile station from a base station is distributed into several fractions due to influence of buildings and the like. Therefore, there are a plurality of radio waves reaching to the mobile station. Respective radio waves causes delay.
In order to realize the CDMA communication, a path search function for recognizing a plurality of radio waves distributed by correlators of respective base stations and a Rake function for combining data of a plurality of finger circuits, each of which performs dispreading for a plurality of distributed radio waves are necessary.
When the mobile station moves, a hand over may be performed. The hand over includes a diversity hand over, in which cells are switched, and a soft hand over, in which sectors are switched.
As shown in FIG. 8, the diversity hand over, in which the cells are switched, is a switching operation to be caused when a device 100 as the mobile station is positioned at a position where a cell 61 and a cell 62 overlap, for switching from a condition receiving a transmission signal of the base station of lower reception power to a condition receiving a transmission signal of the base station of higher reception power. For example, an operation, in which the device 100 as the mobile station switches receiving condition from a condition receiving a transmission signal from a base station 61a corresponding to the cell 61, to a condition receiving a transmission signal from a base station 62a corresponding to the cell 62, is the diversity hand over.
In this case, since a plurality of antennas receive signals simultaneously, reception state of the device 100 is switched from a receiving condition of the transmission signal of the base station 61a to a receiving condition of the transmission signal of the base station 62a without momentaneous interruption of communication. Here, xe2x80x9ccellxe2x80x9d is a area where the transmission signal from the base station reaches. By presence of a plurality of cells, mobile communication in wide range can be realized.
In contrast to this, the soft hand over, in which the sectors are switched, is operation for switching receiving condition when the device 100 as the mobile station is located at a position where a sector 601 and a sector 602 overlap, from a condition receiving a transmission signal of an antenna of lower reception power to a condition receiving a transmission signal of an antenna of higher reception power, as shown in FIG. 9. For example, the operation for switching from the condition receiving the transmission signal from an antenna ANT1 corresponding to the sector 601 to the condition receiving the transmission signal from an antenna ANT2 corresponding to the sector 602.
In this case, since the signals are received from a plurality of antennas simultaneously, communication can be switched from the receiving condition of the transmission signal of the antenna ANT1 to the receiving condition of the transmission signal of the antenna ANT2 without momentaneous interruption. Here, xe2x80x9csectorxe2x80x9d is a range of reaching the transmission signal from one antenna of the base station 60a. Assuming that three antennas ANT1 to ANT3 are provided for one base station 60a, three sectors 601 to 603 are present within the cell of the base station. In this case, by differentiating directionalities of the three antennas, three sectors are provided within the same cell.
Operation of the mobile station upon performing the foregoing diversity hand over and soft hand over (which may be referred to as xe2x80x9chand overxe2x80x9d as generally referred to), will be discusses with reference to the drawings. FIG. 1 is a block diagram showing an internal construction of the mobile station. In FIG. 1, there is shown a construction of the major part of the mobile station upon diversity hand over.
The device 100 as the mobile station receives transmission signals 3 and 4 of radio wave from the base stations 61a and 62a by means of an antenna 5. The received signals 3 and 4 are converted into a chip rate by a radio portion 6. The transmission signal of the base station 61a converted into the chip rate is despread by a correlator 7 for the base station 61a and the radio wave of the base station 62a is despread by a correlator 8 for the base station 62a. These correlators 7 and 8 have function for shifting a phase for a signal which is received and demodulated, per a given interval, and establish correlation between the phase shifted signal and a known data (spread code). Then, the correlators 7 and 8 output delay profile data.
For the signals of respective base stations 61a and 62a despread in the correlators 7 and 8, peak detection of a signal level (power) is performed by a searcher circuit 9 for recognizing a plurality of distributed signals.
A plurality of recognized signals are assigned to a finger circuit 10 for the base station 61a and a finger circuit 11 for the base station 62a. Then data after dispreading by respective finger circuits 10 and 11 are combined by a Rake circuit 12 for outputting a decoded data 13.
Here, an internal construction of the typical searcher circuit 9 is shown in FIG. 10. Referring to FIG. 10, the searcher circuit 9 is constructed with a peak detection circuit 91a for detecting a peak value with respect to the delay profile data provided corresponding to the correlator 7, a peak detection circuit 91b for detecting a peak value with respect to the delay profile data provided corresponding to the correlator 8, and assigning portion 92 for assigning the peak values detected by the peak detections circuits 91a and 91b to the finger circuits 10 and 11. With such construction, the peak values of the delay profile data for the reception signal is assigned to the finger circuit.
Here, if a plurality of radio waves separately arriving should have the same interference if transmitted from the same radio wave transmission source. However, when radio waves transmitted from different radio wave transmission sources are to be combined as in the hand over, magnitudes of interference should be different at different radio wave transmission source. For example, magnitudes of interference are differentiated in such a manner that the radio wave transmitted from one transmission source is as illustrated in FIG. 11A and the radio wave transmitted from the other transmission source is as illustrated in FIG. 11B.
Consideration is given for the case where the reception signals shown in FIGS. 11A and 11B are combined in the Rake circuit 12. In this case, if the reception signals are combined by simple addition and if the arriving signal levels are different, the signal of lower reception power may be extinguished by the signal of higher reception power. Namely, since the peak value 15 of the base station 61a is lower than or equal to a noise level of the base station 62a, the signal from the base station 61a is extinguished as shown in FIG. 12. Then, the finger circuit, output of which is not used, can be present to waste the power consumed by such finger circuit.
On the other hand, since the base station spreads the transmission signal with a unique spread code, if the output data of the correlators of the base stations 61a and 62a are combined, it becomes impossible to identify the base station corresponding to the peal value as shown in FIG. 12. This make the searcher circuit 9 impossible to identify the base station to be assigned for the finger circuit.
In order to solve this problem, it can be considered to perform Rake maximum ratio combination for combining the output data of the correlators at equal noise levels from respective transmission source in the Rake circuit. By such Rake maximum ratio combination, each peak values will never be extinguishede as shown in FIG. 13. Thus, outputs of all of the finger circuits can be combined without wasting.
However, in order to perform Rake maximum ratio combination, it becomes necessary to derive a noise level per the base station and to perform conversion of the signal level for achieving the equal noise level. Then, process becomes complicate and circuit scale is increased, and power consumption is also increased.
The present invention has been worked out in view of the problems set forth above. It is therefore an object of the present invention to provide a CDMA mobile communication system and a CDMA mobile communication method which can reduce power consumption.
According to the first aspect of the present invention, a CDMA mobile communication device for realizing a CDMA communication by combining outputs of finger circuits performing despreading for reception signals, comprises:
subtracting means for performing subtraction between delay profile data respectively output from at least two correlators respectively corresponding to reception signals transmitted from mutually different antennas; and
assigning means for assigning the finger circuits for despreading reception signals to result of subtraction having signal level higher than and lower than a reference level so that outputs of the assigned finger circuits are combined.
Assigning means suppress assignment of the finger circuit for the peak value when noise levels of the delay profile data output from said at least two correlators are mutually different and when the peak value of one delay profile data is lower than a noise level of the other delay profile data. The mutually different antennas are provided in the same base station or the different base station respectivery.
According to the second aspect of the present invention, a CDMA mobile communication device for realizing a CDMA communication by combining outputs of finger circuits performing despreading for reception signals, comprises:
subtracting means for performing subtraction between delay profile data respectively output from at least two correlators respectively corresponding to reception signals transmitted from mutually different antennas;
peak value detecting means for detecting peak values of the result of subtraction; and
assigning means for assigning the finger circuits for despreading reception signals to the peak values of result of subtraction having signal level higher than and lower than a reference level so that outputs of the assigned finger circuits are combined.
Assigning means suppress assignment of the finger circuit for the peak value when noise levels of the delay profile data output from said at least two correlators are mutually different and when the peak value of one delay profile data is lower than a noise level of the other delay profile data.
The subtracting means, the peak value detecting means and the assigning means are integrated into a single chip. The mutually different antennas are provided in the same base station or the different base station respectivery.
According to the third aspect of the present invention, a searcher circuit for a CDMA mobile communication device for realizing a CDMA communication by combining outputs of finger circuits performing despreading for reception signals, comprises:
subtracting means for performing subtraction between delay profile data respectively output from at least two correlators respectively corresponding to reception signals transmitted from mutually different antennas;
peak value detecting means for detecting peak values of the result of subtraction; and
assigning means for assigning the finger circuits for despreading reception signals to the peak values of result of subtraction having signal level higher than and lower than a reference level so that outputs of the assigned finger circuits are combined.
According to the fourth aspect of the present invention, a CDMA mobile communication method for realizing a CDMA communication by combining outputs of finger circuits performing despreading for reception signals, comprises:
subtracting step of performing subtraction between delay profile data respectively output from at least two correlators respectively corresponding to reception signals transmitted from mutually different antennas;
peak value detecting step of detecting peak values of the result of subtraction; and
assigning step of assigning the finger circuits for despreading reception signals to the peak values of result of subtraction having signal level higher than and lower than a reference level so that outputs of the assigned finger circuits are combined.
In short, the present invention can avoid wasteful assignment of the finger circuit and whereby can restrict power consumption without causing increasing complexity of the process or the size of the circuit, by assigning the finger circuits for the peak values of the result of subtraction of the delay profile data, only when it is higher than or lower than a reference level.